There is great concern over confidentiality in wireless data exchanges due to the ever increasing use of wireless networks and the vulnerability of a wireless channel to various types of eavesdropping and security threats. Alongside the conventional cryptography algorithms, physical-layer-security was introduced to provide an extra level of protection against malicious attacks.
Physical-layer security uses the random nature of a wireless channel to secure a path for the data transmission. In general, using multiple antennas introduces additional degrees-of-freedom (DoF) in a communication system. These extra degrees of freedom can be used to provide desired secrecy requirements. One strategy recently utilized for providing secrecy in multi-antenna systems is directional modulation (DM).
Directional modulation is a transmission side process. It enables the transmission of a confidential message towards a certain pre-specified direction, while transmitting random patterns along other directions. The DM algorithm uses a set of data-driven attenuators and phase-shifters along with antenna arrays, in order to have such control over the transmitted pattern. Algorithms have been proposed for single direction transmission, multi-direction transmission, and a multi-user multi-path-based system. All these techniques, however, focus on optimizing secrecy features of directional modulation, while leaving the beam-shape to be decided based on the physical structure of the transmission antenna array.
Moreover, research in the spatial domain has inspired great interest, especially with consideration of massive multiple-input, multiple-output (MIMO) and hybrid beam forming as enabling technologies for 5G networks. The sparse nature of wireless channels in the future-targeted mm-wave frequency range initiated a new point of view towards conventionally used channel models. This new perspective encouraged the deployment of large antenna arrays and beam-forming algorithms.
However, the aforementioned fixed structure for directional modulation may not be well suited for the expected highly dynamic 5G networks. Moreover, the prior multi-user scheme adopts a high complexity structure and the use of zero-forcing algorithms.